The present invention relates to stators for electrical machines (e.g., motors and generators), and in particular to stators that have coolant passageways or ducts.
It is known to produce laminated stators for electrical machines by pressing stacks of annular laminations together. The laminations are typically formed from sheets of electrical grade steel which are usually provided with insulating coatings. Each annular lamination may be formed of a single member or may itself be of segmental construction with the segments abutted against each other e.g., at generally radially extending edges.
The laminations can define axially extending teeth that define therebetween axially extending slots for receiving the conductors of a stator winding. The teeth are circumferentially spaced around the stator surface and carry the magnetic flux that links from the stator to the rotor across the airgap. The conductors are electrically insulated from the teeth.
One of the problems faced by designers of electrical machines is the heat that is created as a result of the various losses, e.g., resistive losses in the stator winding, eddy current losses in the laminations etc. The problem of heat can be particularly acute when trying to design an electrical machine with high power density. The maximum power output of an electrical machine, for a given amount of conductive material (e.g., copper for the stator winding conductors and iron for the magnetic circuit) is limited by the efficiency of the cooling because, if heat is not removed efficiently, the temperature of the electrical machine will increase to a point that can cause the insulation material or some other part of the machine to fail.
Electrical machines can be cooled in a variety of different ways, e.g., direct liquid or air cooling, cooling by conduction to the laminations which are in turn cooled by direct cooling or an external water jacket. In one arrangement, the stator can include one or more axially extending coolant passageways (or ducts) through which a coolant fluid flows in use. Each lamination can have one or more opening formed therein, each opening defining part of an axially extending coolant passageway when the laminations are stacked together. When the stacked laminations are compressed and subjected to appropriate treatment including vacuum pressure impregnation (VPI) and curing, each coolant passageway should be fluid tight over the operational lifetime of the electrical machine. In one arrangement, the surface of each coolant passageway is defined by the surfaces of the aligned lamination openings such that the coolant fluid is in direct contact with the stacked laminations. The shape of each coolant passageway is determined by the shape of the corresponding opening in the laminations. The coolant passageways can have any suitable shape, e.g., a cross section that is substantially circular, oval, rectangular or polygonal, or something more complex such as a star shape which increases the surface area of the coolant passageways and thereby increases the transfer of heat from the stator to the coolant fluid. The shape of the coolant passageways can also be designed to increase heat transfer by promoting turbulent flow of the coolant.
The coolant passageways can be located in the main body of the stator, i.e., the part of the stator core from which the stator teeth extend, or in the stator teeth themselves which is where most of the heat is created during operation of the electrical machine.
During a VPI process, the assembled stator core, which can be pre-heated in a separate process to remove moisture, is placed in a vacuum tank. A suitable resin material (e.g., epoxy) is introduced into the vacuum tank while a vacuum is maintained. The vacuum is held for a period of time before being released. The VPI process fills the gaps between the laminations and bonds the laminations together to form a bonded stator core. After impregnation, the resin material is cured in an oven. The stator core forms part of the stator of the electrical machine.
In practice, the coolant passageways can sometimes be prone to leakage of the coolant fluid even after VPI treatment. In particular, deformations within the stator core can lead the resin material between adjacent laminations to crack. Such deformations can be caused by temperature changes as the electrical machine heats up and cools down during normal operation, or by mechanical loading or vibration, for example. Accordingly, there is a need for an improved way of sealing the coolant passageways to prevent leakage of coolant fluid.